1. Field of the Invention
The present invention relates to a device for recording/reproducing information in/from a record medium such as a DVD-RW (digital video disk rewritable or digital versatile disk rewritable) having a wobble on a track as phase-modulated address information, and relates to a demodulator circuit used in such a device for the purpose of demodulating addresses.
2. Description of the Related Art
In general, there is a need to correctly detect a linear velocity at each radial location of a record medium. To this end, many record media employ a format in which a track is wobbled at the time of manufacturing so as to produce a constant wobble-signal frequency when a CLV (constant linear velocity) control is engaged. A device for recording/reproducing information in/from a record medium detects this wobble signal, and uses the signal for the purpose of controlling rotation of the medium and generating a record clock signal.
Address information is also necessary for the purpose of identifying a record position in a record area where no information has been recorded. In this respect, Japanese Patent Laid-open Application No. 10-69646 discloses a method of phase-modulating track wobbles. Such a phase-modulation scheme may include several different methods. In any methods, however, there is a tradeoff between the amount of recorded information and S/N (signal to noise ratio) of the detected signal. Since a signal obtained from an optical-disk medium has low quality in general, a binary phase-shift keying scheme (BPSK or DPSK: binary modulation using 0° and 180° is the most suitable because it can achieve relatively high S/N.
Demodulation circuits for demodulating this phase-modulated address information (wobbling signal) include an analog demodulator circuit disclosed in Japanese Patent No. 6-19898 and a digital demodulator circuit disclosed in Japanese Patent Laid-open Application No. 5-260413. In these circuits, a carrier wave to which the phase information of a wobbling signal is superimposed is generated from the wobbling signal by a PLL or the like, and a phase difference (0° and 180° in the case of BPSK) is detected to demodulate the information. If a phase error develops between the original carrier wave of the wobbling signal and the carrier wave generated by the PLL, quality of phase demodulation drops, resulting in frequent detection errors.
Such methods and schemes as described above are generally used in the art of cameras and the field of communication. With respect to optical disks, the following problems need to be addressed.
Optical disks produce signals having low signal quality in an area where data has been recorded. To cope with this low signal quality, a wobbling signal is generally processed by a band-pass filter or the like before it is supplied to a PLL or the like to generate a carrier wave. Since such a filter is designed to remove phase modulation components and noise components effectively, a configuration having such a filter tends to frequently develop an inadvertent phase change (phase delay). Further, when access is made t the optical-disk medium under the conditions f constant angular velocity, the carrier-wave frequency of the wobbling signal varies depending n radial positions, so that the phase change (phase delay) caused by the filter as described above is easy to develop. Such a phase change serves as a phase error between the original carrier wave of the wobbling signal and the generated carrier wave, thereby resulting in deterioration of detection accuracy.
Accordingly, there is a need for a demodulation circuit, which can carry out accurate demodulation by properly detecting and correcting a phase error, and a need for a device for recording/reproducing information based on such a demodulation circuit.
Further, when address data is recorded by frequency modulating track wobbles, a carrier frequency of 22.05 kHz with a frequency shift of ±1 kHz is used to represent 0 and 1 of the data.
As previously described, Japanese Patent Laid-open Application No. 10-69646 discloses a method of phase-modulating track wobbles. Since a signal obtained from an optical-disk medium has low quality in general, a binary phase modulation scheme (BPSK or DPSK; binary modulation using 0° and 180°) is most suitable because it can achieve relatively high S/N. This modulation scheme is employed in a system requiring a high S/N, so that more than one carrier-wave cycle is used to represent 1-bit data.
As a demodulation scheme, a scheme disclosed in Japanese Patent No. 6-19898 may be used, which teaches a demodulation device that extracts a carrier signal from a phase-modulated signal, and compares phases between the phase-modulated signal and the carrier signal to demodulate the modulated data. Alternatively, a typical phase-demodulation method that may be found in a text book may be used. A BPSK scheme, however, has a drawback in that the amount of information per unit time is smaller in comparison with the QPSK scheme or the like.
Address information on an optical disk is comprised of a synchronization signal representing a reference position of data and a data signal. It is generally desirable to be able to detect the synchronization signal without a need for special processing. Because of this reason, in a phase-modulation scheme, the synchronization signal is defined simply as a predetermined carrier-wave period that has a reversed phase relative to the carrier-wave signal. In order not to reduce the amount of information to be communicated, a carrier-wave period for the synchronization signal is desirably as short as possible.
The utmost requirement is that the data signal be able to be distinguished from the synchronization signal. Further, there is a need to avoid reducing the amount of information communicated via data signals when the data signals are provided in larger numbers than the synchronization signals. In consideration of the above, it is desirable that the data signals do not include a reversed-phase carrier-wave period longer than the synchronization signal, and rather include a shorter period of reversed phase.
When a phase is reversed during a short time period, however, a detected signal tends to have a relatively small signal level and become noisy, thereby more likely to generate errors.
Accordingly, there is a need for a demodulation circuit that can correctly demodulate data signals when phase reversal is applied to short carrier-wave periods, and a need for an information-recording/reproducing device based on such a demodulation circuit.
Moreover, the BPSK scheme has a problem in that the amount f informati n that can be communicated is small. To increase the fficiency as much as possible, the phase f a signal may be changed carrier-wave cycle by carrier-wave cycle. In this case, however, a related-art demodulation circuit may not have a sufficient demodulation capacity to cope with speed of such phase changes. Accordingly, there is a need for a demodulation circuit that can correctly demodulate signals, and a need for an information-recording/reproducing device based on such a demodulation circuit.